With recent implementation of a blue-light light emitting diode in practical use, studies are being aggressively made to develop a white-light light emitting diode device using this diode as the light emission source. The white-light light emitting diode device is greatly advantageous in that the power consumption is low and the life is long as compared with existing white light sources, and the demand therefore is expected to rapidly expand in the future. As for the method of converting blue light of the blue-light light emitting diode into white light, a method described, for example, in Japanese Unexamined Patent Publication (Kokai) No. 2000-208815 is most commonly employed, where a coating layer capable of absorbing a part of blue light and emitting yellow light nearly in a complimentary color relationship with the blue light and a mold layer for mixing blue light of the light source with yellow light from the coating layer is provided at the front of a light emitting element. The coating layer employed in the conventional technique is provided by coating a mixture of cerium-activated YAG (YAG means yttrium-aluminum-garnet; hereinafter the activated YAG is simply referred to as “YAG:Ce”) powder and epoxy resin on a light emitting element (see, Kokai No. 2000-208815). The light of the light emitting diode used in this technique is at a wavelength in the vicinity of 460 nm. This wavelength is employed because the light emission efficiency of the cerium-activated YAG becomes high in this wavelength region. However, the color of the YAG:Ce photoluminescence is present approximately at x=0.41 and y=0.56 in the CIE chromaticity coordinates (1964) and when this is mixed with the color of excited light at 460 nm, the color produced is not white but becomes white mixed with green-blue color. In order to solve this problem of poor color tone, for example, a method of changing the peak wavelength of YAG:Ce photoluminescence from 530 nm to a longer wavelength is used. However, this method is disadvantageous in that the kinds of elements added are increased and, therefore, not only the adjustment of the composition or the production procedure becomes complicated but also a product having a uniform composition can be hard to obtain.
Accordingly, another method for solving the problem may be considered where a violet color which is a preferred complimentary color of the photoluminescent color of the YAG:Ce phosphor powder is employed as the excitation wavelength. However, use of a violet color causes extreme reduction in the light emission efficiency of the YAG phosphor and, as the emission of a yellow color used for color mixing decreases, a white color light can be hardly obtained. When the YAG:Ce phosphor powder is mixed in a large amount in the coating phase so as to compensate for the reduction of light emission efficiency, the transmission of light is seriously decreased and it is difficult to obtain a bright white-light light emitting diode.
It could therefore be advantageous to provide a technique capable of fabricating a white-light light emitting device assured of good color tinge, without requiring compositional adjustment of the color tone of the Y3Al5O12:Ce phase by a violet color which is in an exactly complementary color relationship with photoluminescence of a ceramic composite material for color conversion comprising a cerium-activated Y3Al5O12.